Jeff, the best time to invest is when you have the money to do so. No one can predict the market. WE can only look at events, economic, political etc and try to adjust our risk accordingly. IF you are going this alone ,without an advisor i would suggest that you educate yourself before you invest. That's going to take some time and effort. And, quite frankly asking investment questions on a metal detecting forum isn't the right way to go about it. You will get many well meaning answers. Most of them uninformed. And, if you do follow a recommendation of a forum member and it goes wrong, it is unlikely you will be able to hold that member responsible. It's your money, you're a big boy, you should have known better. That's not to say the suggestion LL has given you is a bad idea. Only that what has worked for others may not work for you. Their success is based on exactly when they invested, for how long, and in what. One thing is for sure, what's past is past. What does the future hold? That future is where your money will be invested, not the unrepeatable past.
Regarding worry over investing at the wrong time in a market cycle. There is a strategy known as DCA. it's a very basic strategy. DCA is dollar cost averaging. DCA is feeding money into a portfolio over a period of time rather than all at once. If you've ever contributed to a 401k you've already employed DCA. The primary benefit of DCA is that it makes you a correct investor. Feeding in equal periodic amounts will buy you less shares when prices are high and more shares when prices are low. Even without the 50k to invest, do this out of cash flow month after month year after year and it is the road to riches.
Be aware that when you have a lump sum to invest DCA is a double edged sword. You will be happy camper if, while employing DCA ,the market is in a downtrend. But in an uptrend, you will grit your teeth that you didn't go all in. Lastly, If the market is in an extended downtrend it is important to keep feeding the money in. Don't try to out guess the market.